The present invention generally relates to optics and more particularly, to a process of manufacturing optics i.e. optical elements of high accuracy to be used for an optical disc device or the like.
Recently, in the manufacture of optics or optical elements (referred to as optical elements hereinafter), for example, lenses, prisms, etc., instead of polishing a raw material for the optical element such as glass or the like, there have been proposed various processes for molding the optical element by heating and pressurizing the raw material therefor charged in a metal mold. With respect to the glass raw material, although the process for molding under pressure by pouring the glass material in a molten state into the mold is the most efficient, such process is not suitable for the lens formation at high accuracy due to difficulty for controlling contraction of glass during cooling. Therefore, the general practice is such that the glass raw material preliminarily processed into a predetermined configuration is fed between dies for heating and subsequent molding through depression, as disclosed, for example, in Japanese Patent Laid-Open Publication Tokkaisho No. 58-84134.
In the case where a high precision lens is to be produced through molding by one time in the known practice as referred to above, it is necessary to preliminarily process the glass raw material for use in the molding, into a configuration as close to that of a desired lens as possible. Meanwhile, for restricting outer periphery of the lens, a barrel mold is normally employed for the molding, and in the above case, since there is no portion for relieving excessive glass, it is required to accurately keep weight of the glass raw material in agreement with that of the lens to be produced. For the above purpose, there has also been conventionally proposed, for example, in Japanese Patent Laid-Open Publication Tokkaisho No. 59-141435, a method for increasing an allowance in the weight matching of the glass through absorption of extra glass by providing a relief means at part of the barrel mold, and upper and lower dies. Meanwhile, it is also important to make the molded surface of the glass raw material as smooth as possible by effecting a preliminary processing for strictly controlling surface roughness of the glass material in order to obtain a lens with a smooth surface as disclosed in Japanese Patent Laid-Open Publication Tokkaisho No. 60-9716. In the known practices as referred to above, it is required to accurately control the weight of the glass raw material or to provide the relief means in the construction of the mold. However, when such relief means is provided in the construction of the mold, there is such a problem that the molded lens tends to have much portion optically ineffective, thus increasing the weight of the lens. On the other hand, the practice for accurately controlling the weight of the glass material is varied in various ways depending on how the configuration of the glass raw material is determined. By way of example, in the case where the shape of the glass raw material is formed into a glass pre-form approximated to a final product, there may arise such a problem that due to the difficulty in the preliminary processing, 3% of the weight results in a width of scattering, with a rise in cost. Accordingly, for accurately controlling the weight, the columnar shape which may be most easily subjected to preliminary processing at less cost is preferable, but due to the fact that a boundary edge or corner portion between a cylindrical portion of the columnar shape and its top flat face portion is at right angles, there is also a problem that such boundary portion of the molding raw material tends to be chipped during feeding thereof into the mold.
Another molding process taking the place of the polishing method is disclosed, for example, in Japanese Patent Laid-Open Publication Tokkaisho No. 60-246231. Referring to the drawings, the conventional molding method will be explained hereinafter.
Generally, when optical elements are produced by the press molding, the raw material for the optical elements is cut off to a predetermined size so as to be pre-heated close to a glass softening point. Subsequently, the optical element raw material thus pre-heated is fed between upper and lower dies so processed that the raw material is formed into a configuration generally the same as that of a finished optical element, e.g. a finished lens, when said raw material is subjected to mold clamping, thereby to effect pressure molding at a predetermined temperature.
The configuration of the optical element molding raw material should preferably be as simple as possible from the viewpoint of the manufacturing process or processing cost of the material, and for example, there is available a raw material 1 in a columnar shape as shown in FIG. 1 prepared by cutting off at a predetermined width, a rod-shaped raw material processed to have a predetermined outer diameter by a centerless processing. However, when the raw material prepared in the manner as described above is subjected to molding, the edge or corner portions 21 (referred to as corner portions hereinafter) or the raw material 1 are first deformed as shown in FIG. 8, and thus, the portions of the raw material 1 contacting an upper die 12 and a lower die 13 undesirably get to fit said upper and lower dies, thus forming closed spaces 22, which once formed continue to be present up to completion of the molding. Accordingly, the processing surfaces of the dies are not fully transferred onto the raw material 1, thereby resulting in a faulty lens.
A conventional practice for preventing such insufficient transfer fault as referred to above will be explained hereinafter with reference to FIG. 7 showing construction of a known molding apparatus.
In FIG. 7, the molding apparatus is so arranged that a lower die 13 is fixed to a base 13B through a connecting rod 13A, while an upper die 12 is connected to a piston rod 12B through a connecting rod 12A. The optical element raw material 1 is adapted to be heated up to a molding temperature by a heater 18, and when the raw material 1 has reached a desired molding temperature, the upper mold 12 is lowered by hydraulic cylinders 19 to contact the raw material 1. Thereafter, the upper mold 12 is subjected to vertical vibration for pressurization, for example, by means of a servo-pulsar 20 mounted on the apparatus. Such vibrating pressurization is effected, e.g. up to 90% of the total stroke, for molding in the remaining 10% by a steady pressurization. Upon completion of the total pressurizing stroke, energization is suspended, and the mold is opened when the temperature is lowered to a desired level, thereby taking out the finished lens after cooling.
However, in the case where the molding is carried out through employment of the optical element molding raw material as described above, there may arise such defects as formation of cracks or chipping at the corner portions of the raw material which contact the dies when said raw material is charged into the mold. If the molding is effected under the state as described above, there have been many cases where broken raw materials damage the dies, configuration of the finished product becomes insufficient in accuracy or appearance of the finished product becomes poor due to remaining of broken raw materials on the surface of the molded optical element. Meanwhile, since the side face of the optical element forming raw material is a face of a centerless grinding process, the surface roughness thereof is comparatively large, due to consequent deterioration of the surface roughness at part of the optically effective face of the optical element, the transmittance thereof is undesirably deteriorated. Moreover, in the optical element manufacture by the above molding method, the upper die which determines the molded face of the optical element tends to involve air due to repeated contact with and spacing from the optical element in the process of molding, air bubbles are collected in the raw material in the course of molding. Furthermore, during the vibrating pressurization of said upper die, positional alignment with the lower die is very difficult, and therefore, it has been extremely difficult to keep inclination at opposite faces of the optical element within a designed tolerance. Additionally, since the upper die repeats the vibrating pressurization, temperature distribution tends to be non-uniform, with another temperature distribution occurring within the optical element also, and as a result, a mold sink is formed on the optical element, thus making it impossible to fulfill the optical performance.